A polycondensation polymer represented by polyethylene terephthalate resin (hereinafter sometimes referred to as “PET resin”), has excellent features such as heat resistance, weather resistance, mechanical resistance, and transparency. Using such features, PET resin has been widely used in fibers and magnetic tapes as well as beverage containers, preforms for manufacturing beverage containers, injection molded articles for various uses, and extrusion molded articles such as package films and sheets.
However, in molding such a polycondensation polymer, a large amount of debris is produced. For example, defective parts, which are not used as molded articles, obtained during a molding step such as burr of a molded article, runner and sprue generated in injection molding, deckle edge generated in molding sheet and film; and defective parts, which are not used as products, obtained in a manufacturing step of a molded article, such as alternative products in the middle of replacing existing products with the alternative products, defective bottles such as pinhole bottles, nonstandard products, purge products until the quality is stable, and resin pellets. Furthermore, in producing a polycondensation polymer, a large amount of a defective polycondensation polymer that is not used as a product is generated. Examples of such a defective polymer include alternative products in the middle of replacing existing products with the alternative products, nonstandard products, and purge products. Moreover, molded articles on market are recovered as recyclable products. It has been desired to develop a process for recycling these recovered products from a recent environmental protection point of view.
For example, attempts have been made to reuse a recovered resin by compounding a new polyester and a scrap polyester at a point of time during a polyester production step and returning the scrap component to a flow of the polyester production step (for example, see Patent Document 1).
However, a polycondensation polymer such as PET resin has a problem in that its polymer chain is cleaved once heat is applied thereto, decreasing in molecular weight. Also, a recovered resin decreases in molecular weight and thus low in physical properties. Therefore, it is not preferable to employ it again for its original use. For this reason, the resin thus recovered is merely used for food trays, which may be made of a relatively low-molecular weight material and requires less physical properties.
As one of techniques for continuous melt polymerization of esters, a method of conducting polymerization while dropping a prepolymer by gravity from the top of a polymerization reactor. For example, as a process for producing polyesters, there is a technique to supply a PET oligomer having an average degree of polymerization of 8 to 12 (corresponding to a limiting viscosity of 0.1 dl/g or less) at 285° C., drop it by gravity along a cylindrical metal net put perpendicular inside the reactor and conduct polymerization with a reduced pressure inside the reactor (see Patent Document 2) and as a process for producing polyamides or polyesters, there is a technique to conduct polymerization while dropping a polymer along a linear support put perpendicular inside a reactor (see Patent Document 3). See Patent Documents 4 to 7. However, according to the present inventors' study, it is revealed that even if the above techniques are used as such, it is not possible to obtain polyesters having a high degree of polymerization. Further, there is a problem that since oligomers ejected from a porous plate or the like are vigorously foamed to foul the surface of the porous plate or the inner walls of the reactor, so that the fouling is decomposed and modified to be mixed with the polymer during a long time operation and so deteriorates the quality of polyester products. Even if the scrap component having a quality lowered through a thermal history is returned to the flow during the polymerization step by these techniques, it is impossible to obtain polyesters having a high degree of polymerization and also the resulting polyesters are not practically applicable at all because of remarkably deteriorated hue of the product.
Furthermore, attempts have been made to reuse a recovered resin as a raw material by completely decomposing the resin into monomer units (for example, see Patent Document 8). However, the depolymerization of a resin into monomer units must be performed in supercritical conditions in an organic solvent and also requires a purification step of the crystallization solvent on recovered monomers. Therefore, cost inevitably increases.
In the circumstances, it has been desired to develop a simple and inexpensive recycling process for a recovered polycondensation polymer by increasing the molecular weight of the polymer.
[Patent Document 1] JP-B-63-46089
[Patent Document 2] JP-B-58-8355
[Patent Document 3] JP-A-53-17569
[Patent Document 4] U.S. Pat. No. 3,110,547
[Patent Document 5] JP-B-4-58806
[Patent Document 6] WO 99/65970 A
[Patent Document 7] JP-A-58-96627
[Patent Document 8] JP-A-2003-147121